1. Field of the Invention
The present invention relates to a multi-piece solid golf ball with at least a four-layer construction.
2. Prior Art
Golf balls having a variety of constructions are available today on the market. Of these, the golf balls generally used for competitive play are either two-piece solid golf balls having a rubber-based core enclosed within a cover made of ionomer resin or the like, or thread-wound golf balls comprising a solid or liquid center about which is wound a rubber thread which is in turn enclosed within a cover.
Most golfers of ordinary skill use two-piece solid golf balls because of their excellent flight performance and durability. However, as compared with thread-wound golf balls, the two-piece solid golf balls have a very hard feel when hit, and are difficult to control because of the rapid separation of the ball from the head of the club.
This situation has prompted efforts to approximate the feel of a thread-wound golf ball in a solid golf ball. As a result, a number of soft, two-piece solid golf balls have been proposed. A soft core is used to obtain such soft two-piece solid golf balls, but making the core softer lowers the resilience of the golf ball, compromises flight performance, and also markedly reduces durability. As a result, not only do these balls lack the excellent flight performance and durability characteristic of ordinary two-piece solid golf balls, but they are often in fact unfit for actual use.
Ionomer base resin materials are often used as the cover of golf balls. A variety of proposals have been made as to improvements in the resilience of ionomer resins. For example, JP-A 96771/1992, 114124/1994, and 80718/1994 disclose ionomer resins having higher contents of .alpha.,.beta.-unsaturated carboxylic acid (simply referred to as acid contents) than prior art ionomer resins. In general, as the acid content increases, the ionomer resin has a higher hardness, a higher rigidity and greater resilience, but becomes brittle. When the ionomer resin is used as the cover stock of golf balls which is subject to substantial deformation, the durability against repetitive shots is markedly exacerbated. As a result of becoming harder and losing friction, the spin performance upon approach shots is markedly exacerbated. The hard cover gives a hard feel when hit, especially on putting and approach shots. As one solution to these drawbacks, JP-A 92372/1983 and 24085/1995 disclose a two-layer cover structure wherein a soft resin overlies a hard resilient resin for taking advantage of the resilience of the inner layer without detracting from spin performance and soft hitting feel. However, the soft outer/hard inner two-layer cover is susceptible to scuff damages by friction between the club face and the cover upon iron shots.